Cantilevered probe detector with piezoelectric element

ABSTRACT

A disclosed chemical detection system for detecting a target material, such as an explosive material, can include a cantilevered probe, a probe heater coupled to the cantilevered probe, and a piezoelectric element disposed on the cantilevered probe. The piezoelectric element can be configured as a detector and/or an actuator. Detection can include, for example, detecting a movement of the cantilevered probe or a property of the cantilevered probe. The movement or a change in the property of the cantilevered probe can occur, for example, by adsorption of the target material, desorption of the target material, reaction of the target material and/or phase change of the target material. Examples of detectable movements and properties include temperature shifts, impedance shifts, and resonant frequency shifts of the cantilevered probe. The overall chemical detection system can be incorporated, for example, into a handheld explosive material detection system.

CROSS REFERENCE TO RELATED APPLICATION

This is the U.S. National Stage of International Application No. PCT/US2005/035216, filed Sep. 30, 2005, which was published in English under PCT Article 21(2), which in turn claims the benefit of U.S. Provisional Application No. 60/614,592, filed Oct. 1, 2004. Both applications are incorporated by reference herein in their entireties.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with Government support under Prime Contract No. DE-AC05-00OR22725 awarded to UT-Battelle, LLC, by the U.S. Department of Energy and a subcontract awarded to the University of Nevada, Reno by UT-Battelle, LLC. The Government may have certain rights in this invention.

FIELD

This disclosure relates generally to chemical detection, and more particularly to systems and methods for sensing target materials, such as explosive materials, using cantilevered probes.

BACKGROUND

The burgeoning market for explosives screening equipment and an increase in research on chemical and explosive detection technologies are in response to the greater need to perform real-time detection of undesirable chemicals and hidden explosives, such as those concealed in luggage, shipping containers, land mines, and unexploded ordinances. The market for devices that screen people for explosives and various types of biological, chemical or nuclear/radiological weapons is estimated by Homeland Security Research Corp. to reach $3.5 billion by 2006 and $9.9 billion by 2010.

Among the wide range of materials from which explosives can be made are organic nitrates, organonitro compounds, ketone and acyl peroxides, inorganic chlorates, perchlorates, nitrates, fulminates, and acetylides. Some of the explosive residue chemical compounds for detection and identification include 2,4,6-trinitrotoluene (TNT), 2,4,6,n-tetranitro-n-methylaniline (Tetryl), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), pentaerythritol tetranitrate (PETN), glycerol trinitrate (nitroglycerin), and ethylene glycol dinitrate (EGDN).

Many obstacles remain for scientists and engineers working to develop equipment and processes for detecting explosives. Dogs continue to be the preferred explosive detectors, yet widespread deployment of canine teams is neither practical nor cost effective. Moreover, currently available non-canine explosive sensor equipment tends to be complex, bulky, and expensive, and cannot be miniaturized easily.

Currently available explosive and bomb detection systems typically absorb particulate or vapor matter onto a surface, and analyze the matter using techniques such as ion mobility spectrometry (IMS), mass spectroscopy, nuclear magnetic resonance analysis, and gas chromatography. Successful explosive and chemical detection techniques can require sensitivity as low as parts per trillion to parts per quadrillion, in that small explosive devices such as anti-personnel land mines may be constructed from plastic and other non-metallic substances having low vapor pressures. One exemplary explosive and chemical detection system used in airports exposes luggage to a stream of air that dislodges chemicals into the air as vapors, which are subsequently concentrated to create detectable levels of the chemicals. Unfortunately, many conventional explosive and chemical detection systems still have high false alarm rates, slow throughput, operator dependences, and high transaction costs.

Some conventional explosive detection systems include cantilevered elements. One example of this type of system is described by Thundat in “Microcantilever Detector for Explosives,” U.S. Pat. No. 5,918,263, issued Jun. 29, 1999 (Thundat). As disclosed in Thundat, explosive gas molecules that have been adsorbed onto a microcantilever are subsequently heated to cause combustion, which in turn causes bending and a transient resonance response of the microcantilever. Movement of the microcantilever is detected by a laser diode, which is focused on the microcantilever, and a photodetector, which detects deflection of the reflected laser beam caused by a heat-induced deflection and resonance response of the microcantilever. Conventional explosive detectors that include cantilevered elements, such as the detector disclosed in Thundat, have a variety of limitations. For example, many such detectors cannot be miniaturized because they require external cantilever actuation and external sensing.

SUMMARY

Disclosed herein are embodiments of a chemical detection system for detecting a target material, such as an explosive material. Some of these embodiments have potential as extremely sensitive yet inexpensive sensors that can be mass-produced, thereby enabling large-scale sensor deployment. For example, some embodiments may offer several orders of magnitude greater sensitivities when compared to other micro-electrical-mechanical systems (MEMS) such as quartz crystal microbalances (QCM), flexural plate wave oscillators (FPW), and surface acoustic wave devices (SAW).

Embodiments of the disclosed chemical detection system can include, for example, a cantilevered probe, a probe heater thermally coupled to the cantilevered probe, and a piezoelectric element disposed on the cantilevered probe. The piezoelectric element can be configured to detect the target material by a variety of processes, such as by detecting bending, vibrations, recoil, or other movements of the cantilevered probe, a temperature change of the cantilevered probe, an impedance shift of the cantilevered probe, or a resonant frequency shift of the cantilevered probe. In some embodiments, the piezoelectric element is configured to actuate movement of the cantilevered probe. This movement can be useful in the detection process, such as to detect a resonant frequency shift of the cantilevered probe.

The piezoelectric element can include a piezoelectric film disposed on a surface of the cantilevered probe. In some embodiments, the piezoelectric element comprises zinc oxide, lead zircanate titanate, aluminum nitride, a piezoelectric material, or a derivative or combination thereof. The piezoelectric element also can comprise a pyroelectric material. In some embodiments, the probe heater includes piezoresistive element formed in the cantilevered probe, a heater element disposed on the cantilevered probe, or both.

Embodiments of the disclosed chemical detection system can include a variety of additional elements. Some embodiments include an interface circuit electrically coupled to the piezoelectric element. For example, some embodiments include an interface circuit comprising the piezoelectric element as a bridge element in an AC bridge circuit. This AC bridge can be tuned, for example, to one of an on-resonance condition or an off-resonance condition to detect the target material. Embodiments of the disclosed chemical detection system also can include a thermally conductive mesh substantially surrounding the cantilevered probe. This can be useful, for example, to limit the egression of thermal energy liberated by exothermic reactions, such as the deflagration of explosive materials. Embodiments of the disclosed chemical detection system also can include a mechanical stop configured to contact the cantilevered probe. This stop can be used, for example, to detect movement of the cantilevered probe. Some embodiments are handheld. These and other embodiments can include an enclosure, such as a handheld enclosure.

Examples of target materials that can be detected by some embodiments of the disclosed chemical detection system include 2,4,6-trinitrotoluene, 2,4,6,n-tetranitro-n-methylaniline, 1,3,5-trinitro-1,3,5-triazacyclohexane, 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane, pentaerythritol tetranitrate, glycerol trinitrate, ethylene glycol dinitrate, and derivatives and combinations thereof. To aid in the detection of trace concentrations, some embodiments include a target material concentrator coupled to the cantilevered probe. These and other embodiments also can include a selective coating disposed on at least a portion of the cantilevered probe, the selective coating being configured for selective adsorption of the target material. In these and other embodiments, additional selectivity can be achieved by fabricating multiple cantilevered probes in a cantilevered probe array. In these arrays, at least two of the cantilevered probes can be frequency-differentiated. For example, at least one cantilevered probe can be tuned to an on-resonance condition while at least one other cantilevered probe is tuned to an off-resonance condition. Multiple cantilevered probes in a cantilevered probe array, such as frequency-differentiated cantilevered probes, can be connected in series.

Also disclosed are embodiments of a method of detecting a target material, such as an explosive material. These embodiments can include, for example, exposing a cantilevered probe to a carrier including a target material such that the target material is transferred onto the cantilevered probe. Some embodiments also include heating the cantilevered probe, such as to a temperature sufficient to cause the target material to undergo a phase change or to a temperature sufficient to cause the target material to deflagrate or ignite. These and other embodiments also can include piezoelectrically detecting a movement of the cantilevered probe or a property of the cantilevered probe, such as by generating an electrical signal with a piezoelectric element connected to the cantilevered probe. The same or another piezoelectric element also can be used to piezoelectrically actuate a movement of the cantilevered probe. In some embodiments, piezoelectric elements are driven by a variable-frequency drive voltage, such as to isolate the responses of frequency-differentiated cantilevered probes.

In some disclosed embodiments, the movement of the cantilevered probe or a change in the property of the cantilevered probe is caused by transferring the target material onto the cantilevered probe. Some disclosed embodiments also include causing the target material to undergo a phase change or a reaction. This phase change or reaction also can cause the movement of the cantilevered probe or a change in the property of the cantilevered probe. Also among the movements and property changes that can be piezoelectrically detected are resonant frequency shifts, cantilever bending, thermal signatures, recoil responses, pyroelectric charge generation, impedance shifts and temperature shifts. Based on the movement of the cantilevered probe or a change in the property of the cantilevered probe, some embodiments also include identifying the target material. Identifying the target material can involve comparing a cantilevered probe response to a reference cantilevered probe response.

Also disclosed are embodiments of a method for making a chemical detection system. These embodiments can include providing a cantilevered probe, providing a probe heater thermally coupled to the cantilevered probe, and providing a piezoelectric element disposed on the cantilevered probe. In some of these embodiments, providing the piezoelectric element includes depositing a piezoelectric film on a surface of the cantilevered probe. These and other embodiments also can include depositing a selective coating on a surface of the cantilevered probe. More than one cantilevered probe can be assembled to form a cantilevered probe array.

Various embodiments are illustrated in part by the accompanying drawings and the detailed description given below. The drawings and the detailed description should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding. Furthermore, the drawings are not drawn to scale. The drawings and the detailed description are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a chemical detection system for detecting at least one explosive material, in accordance with some embodiments of the current invention;

FIG. 2 is a perspective view of a self-sensed cantilevered probe array, in accordance with some embodiments of the current invention;

FIG. 3 is a plan view of a pair of cantilevered probes with probe heaters and piezoelectric detectors, in accordance with some embodiments of the current invention;

FIG. 4 is a schematic diagram of a system for detecting an explosive material, in accordance with some embodiments of the current invention;

FIG. 5 a, FIG. 5 b and FIG. 5 c are graphs showing characteristic resonant frequencies of a cantilevered probe prior to exposure to an explosive material, after exposure to the explosive material, and after deflagration or ignition of the explosive material, respectively, in accordance with some embodiments of the current invention;

FIG. 6 a, FIG. 6 b and FIG. 6 c are elevation views showing characteristic bending of a cantilevered probe prior to exposure to an explosive material, after exposure to the explosive material, and after deflagration or ignition of the explosive material, respectively, in accordance with some embodiments of the current invention;

FIG. 7 a, FIG. 7 b and FIG. 7 c are elevation views showing bending and vibrations of a cantilevered probe prior to exposure to an explosive material, after exposure to the explosive material, and after deflagration or ignition of the explosive material, respectively, in accordance with some embodiments of the current invention;

FIG. 8 a, FIG. 8 b and FIG. 8 c are graphs showing periodic heating of a cantilevered probe prior to exposure to an explosive material, during exposure to the explosive material, and after deflagration or ignition of the explosive material, respectively, along with a generated piezoelectric detector output signal, in accordance with some embodiments of the current invention;

FIG. 9 is a plan view of an array of cantilevered probes with an explosive material concentrator surrounding the cantilevered probe array, in accordance with some embodiments of the current invention;

FIG. 10 is a perspective view of a handheld system for detecting an explosive material, in accordance with some embodiments of the current invention; and

FIG. 11 is a flow chart of a method for detecting an explosive material, in accordance with some embodiments of the current invention.

DETAILED DESCRIPTION

The following terms may be abbreviated in this disclosure: 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), 2,4,6,n-tetranitro-n-methylaniline (Tetryl), 2,4,6-trinitrotoluene (TNT), atomic force microscopy (AFM), central processing unit (CPU), deep reactive ion etching (D-RIE), digital signal processor (DSP), ethylene glycol dinitrate (EGDN), field-programmable gate array (FPGA), flexural plate wave oscillators (FPW), fast Fourier transform (FFT), glycerol trinitrate (nitroglycerin), ion mobility spectrometry (IMS), lead zircanate titinate (PZT), local area network (LAN), micro-electrical-mechanical systems (MEMS), pentaerythritol tetranitrate (PETN), quality (Q), quartz crystal microbalances (QCM), silicon-on-insulator (SOI), surface acoustic wave devices (SAW), universal serial bus (USB), and wide area network (WAN).

Unless otherwise explained, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The singular terms “a,” “an,” and “the” include plural referents unless context clearly indicates otherwise. Similarly, the word “or” is intended to include “and” unless the context clearly indicates otherwise. The terms “comprises” and “includes” are equivalent. The same reference numerals are used throughout the Figures to indicate similar or identical features. U.S. application Ser. No. 10/967,748 is incorporated herein by reference.

Disclosed herein are embodiments of a chemical sensor, embodiments of a method for making the disclosed chemical sensor and embodiments of a method for sensing chemicals. Although not limited by any particular advantages, the disclosed embodiments may have one or more advantages over the prior art. Some disclosed embodiments require detection surface areas that are orders of magnitude smaller than the surface areas required by other types of sensors. In addition, some embodiments are capable of operating in several detection modes, such as mass loading and bending. Most other sensors operate in only a single detection mode. Furthermore, many of the disclosed embodiments can be mass-produced at relatively low cost. For example, silicon cantilevered probes can be manufactured using standard semiconductor manufacturing equipment. Finally, some of the disclosed embodiments have demonstrated superior detection sensitivities in comparison to at least some conventional sensors.

FIG. 1 illustrates a chemical detection system 10 for detecting one or more explosive materials 16 or other target chemical species 12 also referred to as target materials, in accordance with some embodiments of the present invention. As shown, the chemical detection system 10 includes a cantilevered probe 30, a probe heater 36 thermally coupled to the cantilevered probe 30, and a piezoelectric element 32 disposed on the cantilevered probe 30. The piezoelectric element 32 can be configured to detect explosive material 16 adsorbed onto the cantilevered probe 30, such as when the probe heater 36 heats the cantilevered probe 30. In various embodiments, the piezoelectric element 32 can provide a piezoelectric element output signal related to, for example, a resonant frequency shift, cantilever bending, a thermal signature, a recoil response, a pyroelectric charge generation, an impedance shift, a temperature shift, or combinations thereof. Throughout this disclosure, descriptions of the adsorption of explosive material 16 or other target chemical species 12 refer to the attachment or inclusion of such material onto or into the cantilevered probe 30, whether by adsorption, absorption, reaction, or any other form of attachment or incorporation.

The explosive material 16 can include, for example, 2,4,6-trinitrotoluene (TNT), 2,4,6,n-tetranitro-n-methylaniline (Tetryl), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), pentaerythritol tetranitrate (PETN), glycerol trinitrate (nitroglycerin), ethylene glycol dinitrate (EGDN), derivatives thereof, or combinations thereof.

The cantilevered probes 30 can be self-sensing. The chemical detection system 10 can include multiple cantilevered probes 30 arranged in a cantilevered probe array 20. Each cantilevered probe 30 can have one or more suspended cantilevered element and one or more associated piezoelectric element 32. The piezoelectric element 32 can include, for example, a deposited layer of piezoelectric material, such as zinc oxide (ZnO), lead zircanate titinate (PZT), aluminum nitride, a piezoelectric material, or a derivative or combination thereof. The piezoelectric element 32 also can include a pyroelectric material. The piezoelectric element 32 can be configured to bend, deflect or vibrate the cantilevered element when excited or actuated by an applied drive voltage. The piezoelectric element 32 also can be configured to generate a voltage as the associated cantilevered probe 30 bends, deflects or vibrates. In this way, the piezoelectric element 32 can be configured to sense movements of the associated cantilevered probe 30 such as shifts in bending, vibrations, or recoil and/or to actuate movement of the associated cantilevered probe 30. Some embodiments, however, include a separate piezoelectric drive mechanism or other mechanism that drives the cantilevered probe 30.

As mentioned above, the cantilevered probe 30 may be part of a cantilevered probe array 20. The cantilevered probes 30 in the cantilevered probe array 20 may be frequency-differentiated such that cantilevered probes having different effective masses or effective spring constants exhibit different resonant frequencies. The cantilevered probes 30 can be manufactured, for example, with small differences in cantilever lengths, resulting in separations in resonant frequencies that allow the resonant frequency of each cantilevered probe in the cantilevered probe array 20 to be detected with as few as two wires connected to the cantilevered probe array 20. Thus, the cantilevered probe arrays 20 with two or more cantilevered probes 30 can be packaged and connected to an interface circuit 40 with a minimal number of bond pads, interconnection traces and bond wires to external interface and control electronics. The interface circuit 40, which can be coupled to the cantilevered probe array 20, can be configured to actuate and sense movement of the cantilevered probes 30. Parallel arrays of cantilevered probes 30 can be configured with elements that number from a few to a million or more cantilevered probes on one substrate or die. Groups of cantilevered probe arrays 20 may be connected, for example, during on-chip trace definition, while being wire-bonded to a leadframe or package, or at the socket or board level.

Non-overlapping, independent and orthogonal explosive and chemical-sensing effects on individual cantilevered probes 30 in the cantilevered probe array 20 may be desirable but not necessary when many cantilevered probes 30 with various coatings and coating thicknesses are used for detection. Signal processing and pattern recognition of the resonance-frequency data from multiple cantilevered probes 30 may be employed to differentiate between various explosive materials and chemicals in varying concentrations having sometimes small and sometimes null effects. Differentiation between similar chemical substances can be made, and their constituency and concentration can be determined, in a system where a variety of selective coatings 34 are applied to multiple cantilevered probes 30. These selective coatings 34, for example, can be used to selectively adsorb different types of chemicals onto the cantilevered probes 30. The selective coating 34 can be positioned on or around one or more of the cantilevered probes 30 of a cantilevered probe array 20, such as one or more cantilevered probes 30 with or without probe heaters 36, to provide, for example, two or more differentiable output signals for identifying the target chemical species 12.

The chemical detection system 10 can be configured to detect one or more target chemical species 12, such as mercury, hydrogen, an alcohol, water vapor, an explosive material, a chemical element, a chemical compound, an organic material, an inorganic material, a gaseous substance, a liquid, a biological material, a DNA strand, a bioactive agent, a toxin, or derivatives or combinations thereof. Throughout this disclosure, the target chemical species 12 can be any chemical, biological, or explosive material targeted for detection.

Typically, one or more cantilevered probes 30 can be configured to respond when exposed to the explosive material 16. For example, the cantilevered probes 30 may respond when absorbing, adsorbing, or otherwise reacting to the explosive material 16 and/or other target chemical species 12. When the cantilevered probe array 20 is exposed to the explosive material 16 and is actuated by the interface circuit 40, one or more of the cantilevered probes 30 in the cantilevered probe array 20 may exhibit a response, such as a shift in bending, a change in a resonant frequency, an impedance shift, or a shift in temperature. When exposed to the explosive material 16 or other target chemical species 12, the cantilevered probes 30 also may increase or decrease in mass, or become more or less rigid. These and other responses may result in the generation of a piezoelectric element output signal.

In one example, the cantilevered probe 30 comprises a patterned layer of gold. When exposed to mercury, the gold and mercury react to form an amalgam. The gold-mercury amalgam adds mass to the cantilevered probe 30 and therefore tends to decrease the resonant frequency of the cantilevered probe 30. Amalgam formation, however, also increases the mechanical stiffness of the cantilevered probe 30, thereby increasing its natural resonant frequency. These two effects tend to cancel each other, though one effect can be made dominant by careful selection and placement of a chemical-sensitive selective coating 34 on the cantilevered probe 30.

In one exemplary detection mode, the adsorbed explosive material 16 deflagrates or ignites when heated by a probe heater 36 to cause an exothermic reaction, which in turn causes a piezoelectric output signal to be generated by a piezoelectric element 32. For example, the piezoelectric element 32 may generate an electrical charge when the probe heater 36 heats the cantilevered probe 30. The piezoelectric element 32 also may detect an increase in temperature of the cantilevered probe 30 when the exothermic reaction occurs. Alternatively, the explosive material 16 may melt or evaporate when the probe heater 36 heats the cantilevered probe 30 and the phase transformation may be detected with the piezoelectric element 32. Alternatively, the explosive material 16 may be detected by an impedance shift of the piezoelectric element 32 when the probe heater 36 heats the cantilevered probe 30. The reactive portion, the resistive portion or a combination of both may shift in response to heating of the cantilevered probe 30.

In some embodiments, the piezoelectric characteristics of the piezoelectric element 32 may detect a shift in bending of the cantilevered probe 30 as the explosive material 16 is adsorbed onto a surface of the cantilevered probe 30, is desorbed from the cantilevered probe 30, or reacts exothermically on the cantilevered probe 30. For example, as the explosive material 16 accumulates on the cantilevered probe 30, the cantilevered probe 30 may bend upwards or downwards depending on the stress state of the added or removed material. Alternatively or in conjunction, the piezoelectric element 32 may detect a shift in a resonant frequency of the cantilevered probe 30 when the explosive material 16 is adsorbed onto the cantilevered probe 30, is desorbed from the cantilevered probe 30, or reacts exothermally on the cantilevered probe 30. For example, as the explosive material 16 accumulates on the cantilevered probe 30, the resonant frequency may decrease due to the additional mass loading. Similarly, as the explosive material 16 desorbs or is otherwise removed, the resonant frequency may return towards its previous condition prior to mass loading. The effects described above may occur in various combinations. Analysis of one or a multiplicity of these effects may be used to identify an explosive or a non-explosive material absorbed onto cantilevered probe 30.

The probe heater 36, a piezoresistor serving as a temperature sensor, the piezoelectric element 32, or another on-board temperature sensor may be used to indicate the temperature of the cantilevered probe 30, from which the ignition temperature or deflagration temperature of the explosive material 16 may be determined. Alternatively, an onboard temperature sensor may be used to determine the heat of vaporization, melting temperature, phase change, chemical reactions, exothermic reactions, endothermic reactions, or time dependencies thereof associated with the explosive material 16 or other target chemical species 12 to aid in the identification.

As discussed above, one or more selective coatings 34 may be disposed on one or more cantilevered probes 30 in the cantilevered probe array 20 to facilitate chemical detection and specificity. For example, one or more cantilevered probes 30 in the cantilevered probe array 20 may be coated, uncoated, or otherwise treated to detect the explosive material 16. The selective coating 34 may be applied to a portion of one or more of the cantilevered probes 30 in the cantilevered probe array 20. For example, the selective coating 34 may be applied to the topside or bottom side of one or more of the cantilevered probes 30 or to portions thereof. The selective coating 34 can include, for example, an epoxy resin such as Novolac™, a fluoropolymer such as FluoroPel™, a gold layer, a palladium layer, an alcohol-absorbent polymer, a water-absorbent material, a chemical-sensitive polymer, a chemical-sensitive layer, a biosensitive material, a thiol, or derivatives or combinations thereof.

Various application methods can be used to deposit or apply the selective coating 34 and to otherwise treat surfaces of the cantilevered probes 30. The selective coatings 34 can comprise, for example, a dipped coating, a sprayed coating, or a dispensed coating disposed on at least a portion of one or more of the cantilevered probes 30. An exemplary chemical-sensitive selective coating 34 includes a masked coating disposed on a portion of one or more of the cantilevered probes 30. In an alternative application method, a non-homogeneous coating material is applied to a set of cantilevered probes 30 in the cantilevered probe array 20, such that constituents of the non-homogeneous coating material are deposited on the cantilevered probes 30 with suitable variations in composition, coverage, and/or thickness.

In some disclosed embodiments, the chemical detection system 10 includes one or more reference cantilevered probes 30 r in the cantilevered probe array 20. The reference cantilevered probe 30 r can provide a reference cantilevered probe response when the cantilevered probe array 20 is exposed to the explosive material 16 or other target chemical species 12. The reference cantilevered probes 30 r can be formed, for example, with no coating materials disposed thereon to reduce or eliminate sensitivity to the explosive material 16 or other target chemical species 12. Alternatively, the reference cantilevered probes 30 r can have an inert coating disposed on their surface to reduce or eliminate sensitivity to the explosive material 16. Alternatively, one or more reference cantilevered probes 30 r can be mechanically isolated from exposure to the explosive material 16 while other portions of the cantilevered probe array 20 are exposed.

The explosive material 16, which may be located in a liquid or gas carrier 14, such as air, water, low-pressure gas, or plasma, can be transported in a forced or free manner towards the cantilevered probes 30. Once the explosive material 16 makes contact with surfaces of the cantilevered probes 30 it may invoke, for example, shifts in resonant frequency, Q factor, impedance, phase, or deflection amplitudes. Impedance shifts may be obtained, for example, by absorbing explosive material 16 or other target chemical species 12 directly into the bulk of a piezoelectric or pyroelectric film of the piezoelectric element 32. The absorption may be enhanced, for example, by increasing the periphery of unpassivated sidewalls, such as with narrow line widths and small spaces between multiple segments of the piezoelectric element 32.

The cantilevered probe array 20 may be actuated with an excitation voltage applied to a piezoelectric drive mechanism serving optionally as the piezoelectric element 32 disposed on each of the cantilevered probes 30 in the cantilevered probe array 20. To reduce the number of external pads and connections, a group of cantilevered probes 30 may be connected in series and electrically connected to a pair of cantilevered probe array drive pads 24, which may be electrically connected to an interface circuit 40. While this configuration can increase the series resistance of the string, differentiation of individual cantilevered probes 30 may be made by detection of signals at or near the resonant frequency of the selected cantilevered probes 30. Alternatively, a group of cantilevered probes 30 may be connected in parallel and electrically connected to a pair of cantilevered probe array drive pads 24, increasing the effective capacitance and decreasing the effective resistance, while still allowing differentiation of individual cantilevered probe responses based on frequency. Alternatively, cantilevered probes 30 may be connected in a network of series-connected and parallel-connected cantilevered probes with frequency-identifiable addressable elements.

The interface circuit 40 can provide excitation voltages for piezoelectric material on the cantilevered probes 30 and sense deflections and vibrations of the cantilevered probes 30 with the same or a different piezoelectric material. In one example, the interface circuit 40 includes an adjustable frequency generator that is scanned through a predetermined frequency range to excite one or more of the cantilevered probes 30 in the cantilevered probe array 20. In another example, the interface circuit 40 includes an impedance analyzer that is scanned through a resonant frequency of at least one cantilevered probe 30, measuring the magnitude and phase from the cantilevered probes 30 and monitoring for any variations in impedance as the cantilevered probes 30 are exposed to one or more explosive materials 16. In another example, the interface circuit 40 includes an oscillator circuit operating at a resonant frequency of at least one cantilevered probe 30 in the cantilevered probe array 20.

In another example, the interface circuit 40 includes an oscillator circuit operating at a predetermined frequency that is near, yet off-resonance with respect to one or more of the cantilevered probes 30 in the cantilevered probe array 20. This configuration may result in the generation of higher amplitudes of vibration and therefore higher output signals as the resonant frequency of the selected cantilevered probe 30 shifts and moves towards the predetermined frequency. The predetermined frequency may be set, for example, slightly above or slightly below the resonant frequency of one or more of cantilevered probes 30.

In another example, the amplitude of bending and/or vibration is monitored as the cantilevered probe 30 strikes against a fixed or adjustable mechanical stop such as a piezoelectric slab or a piezotube. In another example, the interface circuit 40 includes an impulse circuit for applying an electrical impulse to the cantilevered probe array 20, and the ring-down of the cantilevered probes 30 is monitored. In another example, noise, such as pink noise or white noise, is applied to excite the cantilevered probe array 20. In some embodiments, the interface circuit 40 includes a network analyzer for detecting signals from the cantilevered probe array 20. The interface circuit 40 or a controller 50 may include a fast Fourier transform generator to perform a fast Fourier transform (FFT) on the shifted cantilevered probe response, and to provide respective frequencies of the cantilevered probes 30 in the cantilevered probe array 20, which can be correlated with previously measured probe responses and used to identify the explosive material 16 and/or other target chemical species 12.

As discussed above, the chemical detection system 10 may include an interface circuit 40 electrically coupled to a piezoelectric element 32 that enables the detection of an explosive material 16 and/or a target chemical species 12. The interface circuit 40 may contain, for example, the piezoelectric element 32 as a bridge element in an AC bridge circuit. The AC bridge circuit may be tuned to an on-resonance condition or on off-resonance condition to detect the explosive material 16. In operation, the output of the AC bridge circuit may shift as the resonant frequency of the cantilevered probe 30 moves off-resonance with the addition or subtraction of mass. Alternatively, the output of the AC bridge circuit may shift as the resonant frequency of the cantilevered probe 30 moves towards the off-resonance tuned condition. Off-resonance tuning can allow any signals generated by the piezoelectric element 32 to be distinguished from any output due to vibrations of the cantilevered probe 30. In some embodiments, one or more of the cantilevered probes 30 in the cantilevered probe array 20 are tuned to an on-resonance condition, and one or more other cantilevered probes 30 in the cantilevered probe array 20 are tuned to an off-resonance condition to detect the explosive material 16.

The interface circuit 40 may detect shifted cantilevered probe responses from one or more actuated cantilevered probes 30 in the cantilevered probe array 20. Examples of shifted cantilevered probe responses include a shift in a resonant frequency of one or more of the cantilevered probes 30, a shift in a quality (Q) factor of one or more of the cantilevered probes 30, a shift in impedance of one or more of the cantilevered probes 30, a shift in phase of one or more of the cantilevered probes 30, a shift in deflection amplitude of one or more of the cantilevered probes 30, and combinations thereof. With exposure to the explosive material 16 or other target chemical species 12, one or more cantilevered probes 30 in the cantilevered probe array 20 can exhibit shifts in various properties. Similarly, with exposure to more than one explosive material 16 or other target chemical species 12, one or more cantilevered probes 30 in the cantilevered probe array 20 may exhibit shifts from which multiple explosive materials 16 and/or other target chemical species 12 can be determined.

A controller 50 such as a central processing unit (CPU), a digital signal processor (DSP), a microcontroller, or a field-programmable gate array (FPGA) may be included in the chemical detection system 10 to execute programmed code and provide monitoring, controlling and analyzing functions. The controller 50 can be in electrical communication with the interface circuit 40 and may be located, for example, on a substrate 22 along with the cantilevered probe array 20, within an enclosure 60 on the same circuit board or in the same package as the cantilevered probe array 20, or located remotely with respect to the enclosure 60. The controller 50 may internally contain the functions and capabilities of the interface circuit 40. The controller 50 may receive shifted cantilevered probe responses from a set of one or more of the cantilevered probes 30 in the cantilevered probe array 20.

The explosive material 16 and other target chemical species 12 may be determined based on the shifted cantilevered probe response using, for example, an algebraic model that relates shifts in cantilevered probe responses to explosive materials and concentration. Alternatively, the explosive material 16 may be determined based on a comparison between the shifted cantilevered probe responses and a reference set of cantilevered probe responses. Such reference sets can be obtained by exposing the cantilevered probes 30 to controlled environments with known explosive materials and concentrations during calibration at the factory or on site. The controller 50 can determine one or more explosive material 16, for example, through pattern recognition techniques, statistical processes, or fuzzy logic with comparison to the reference set of cantilevered probe responses. The reference set of cantilevered probe responses can comprise, for example, a learned set obtained from shifts in cantilevered probe responses by cantilevered probes 30 that have been exposed to known explosive materials and concentrations under controlled laboratory or factory environments.

In some embodiments, heating of select cantilevered probes 30 burns off, evaporates off, or otherwise cleans and resets the cantilevered probe 30 to a nascent condition. In these and other embodiments, the probe heater 36 may be coupled to at least one cantilevered probe 30 in the cantilevered probe array 20. The probe heater 36 may be formed, for example, with a resistive layer disposed on the surface of or formed within the cantilevered probe 30, such as by ion implantation. Exemplary probe heaters 36, which may be connected in series or parallel or individually connected, can be formed on one, several, or all of the cantilevered probes 30 within the cantilevered probe array 20. The probe heaters 36 also may be used to react the explosive material 16 on the cantilevered probe 30 by heating the probe to a predetermined temperature where the reaction can occur. Alternatively, the probe heaters 36 may be used to ignite or deflagrate condensate of explosive vapors on the cantilevered probes 30. The probe heater 36 can comprise, for example, a resistive or a piezoresistive element formed in one or more of the cantilevered probes 30 or a heater element, such as a patterned metal film, disposed on a surface of the cantilevered probe 30.

The piezoelectric element 32, which may also serve as a piezoelectric drive mechanism, can comprise, for example, zinc oxide, lead zircanate titanate, aluminum nitride, a piezoelectric material, or derivatives or combinations thereof. The piezoelectric element 32 also can comprise a pyroelectric material. Piezoelectric materials typically expand or contract when driving voltages are applied, and conversely generate a voltage when stressed or compressed. Piezoelectric materials are generally pyroelectric, in that a pyroelectric charge, voltage or current is generated when the material is heated. For example, the piezoelectric element 32 may generate a piezoelectric element output signal when the explosive material 16 in proximity to the piezoelectric element 32 ignites, deflagrates or otherwise generates heat. In some embodiments, the piezoelectric element 32 serves simultaneously as a piezoelectric thermal detector and a piezoelectric drive mechanism to drive and excite the cantilevered probe 30, such as into resonance. In other embodiments, the piezoelectric element 32 is separated from a piezoelectric drive mechanism, such as a piezoelectric drive mechanism that also is located on a surface of cantilevered probe 30.

The chemical detection system 10 may contain one or more cantilevered probe arrays 20 in an enclosure 60, which may include an inlet port 62 and an outlet port 64 for transport of the explosive material 16, the target chemical species 12, and the carrier 14. The explosive material 16 may enter the enclosure 60 through the inlet port 62 and be exposed to the cantilevered probe array 20. The explosive material 16 or byproducts thereof may exit through the outlet port 64. The enclosure 60 also may include filters, scrubbers, and other media treatment elements to aid in the detection of the explosive material 16.

A transport mechanism 66 such as a pump or a fan with ductwork or piping may be included for transporting the explosive material 16 to the cantilevered probe array 20. The chemical detection system 10 also may include an explosive material concentrator 68 coupled to one or more of the cantilevered probes 30. The concentrator 68, such as a pressurizing system or a condenser and heater system, may be included to concentrate the explosive material 16 and/or other target chemical species 12 proximal to the cantilevered probe array 20 to facilitate detection. In some embodiments, the explosive material 16 is concentrated on one or more cantilevered probes 30 when the concentrator 68 is locally heated.

The chemical detection system 10 may include a thermally conductive mesh 58 such as a copper screen or a metal mesh substantially surrounding the cantilevered probes 30 to limit the egression of thermal energy, such as from an exothermic reaction. As the explosive material 16 deflagrates, ignites or otherwise burns, hot air may be generated near the cantilevered probe 30. The thermally conductive mesh 58 may facilitate cooling of the hot air and otherwise limit heat transfer away from the cantilevered probes 30, such as beyond the enclosure 60.

The chemical detection system 10 may be connected to a local area network (LAN), a wide area network (WAN), the Internet, or other networked communication system via one or more wired or wireless connections. The chemical detection system 10 may be installed, for example, into an air handling system of a building or airport that has many inlets, into a standalone unit with a portal for chemical detection, or into a handheld unit for portable use. Moreover, the chemical detection system 10 may be installed in shipping containers and crates during storage and transit for chemical detection and monitoring.

FIG. 2 illustrates a self-sensed cantilevered probe array, in accordance with some embodiments of the present invention. As shown, the self-sensed cantilevered probe array 20 includes a plurality of cantilevered probes 30 on a substrate 22. The cantilevered probes 30 can include piezoelectric elements 32, probe heaters 36, and/or chemical-sensitive selective coatings 34. Variations in length or thickness of the cantilevered probes 30 and variations in the thickness and coverage of the applied coatings may allow for frequency differentiation between the cantilevered probes 30 within the cantilevered probe array 20.

The cantilevered probes 30 may have a rectangular shape, though other shapes may be suitably used, such as pointed cantilevers, V-shaped cantilevers, triangular-shaped cantilevers, dual-arm cantilevers, or balanced cantilevers. The cantilevered probes 30 may be arranged and attached to the substrate 22 in an array in which the cantilevered probes are all identical, all different, or combinations thereof.

In some embodiments, the cantilevered probe array 20 is actuated with an excitation voltage applied to a piezoelectric element 32 that serves as a piezoelectric drive mechanism and as a piezoelectric sense mechanism. In one example, the cantilevered probes 30 are series-connected to a pair of cantilevered probe array drive pads 24 on the substrate 22. The cantilevered probes 30 also can be parallel connected to the pair of cantilevered probe array drive pads 24. The cantilevered probe array 20 also can comprise a network of series-connected and parallel-connected cantilevered probes that connect electrically to the pair of cantilevered probe array drive pads 24. More than one group or array of cantilevered probes 30 may be included on the substrate 22. Additional connections with associated pads may be made to the piezoelectric elements 32 on particular cantilevered probes 30. The substrate 22 also may have through-wafer vias for backside connection to the drive pads 24.

The substrate 22 can include a semiconductor substrate such as a silicon wafer, a silicon-on-insulator (SOI) wafer, a glass substrate, or other suitable substrate for forming the cantilevered probes 30 thereon. The cantilevered probes 30 can comprise materials such as silicon, polysilicon, silicon nitride, zinc oxide, aluminum nitride, metals, pyroelectric materials, piezoelectric materials, or derivatives or combinations thereof. These materials can be present in various forms, such as sheets, films and layers. For example, a zinc oxide, PZT or aluminum nitride film can be deposited on a layer of single-crystal silicon, patterned, and etched. Conductive layers for top and bottom electrodes, interconnections, and probe heater connections then can be deposited and etched accordingly. The cantilevered probes 30 can be defined with a photomask and associated lithographic sequences along with deep reactive ion etching (D-RIE) or anisotropic etching of the cantilevers and substrate. This allows the formation and freeing of the silicon cantilevers with interconnected ZnO electrodes in series, parallel, or series-parallel configurations. Excitation and detection of the cantilevers can occur with voltages applied to the piezoelectric material. The piezoelectric elements 32 may be formed with deposition and patterning processes as are known in the art. The probe heaters 36 on the cantilevered probes 30 can be formed, for example, by selectively implanting portions of the cantilevered probe 30 or by depositing, patterning and etching a metal film on the cantilevered probe 30.

A chemical-sensitive selective coating 34 may be applied to at least a portion of one or more of the cantilevered probes 30. The chemical-sensitive coating 34 can include a material, such as an epoxy resin, a fluoropolymer, gold, palladium, an alcohol-absorbent polymer, a water-absorbent material, a chemical-sensitive polymer, a chemical-sensitive material, a biosensitive material, a thiol, or derivatives or combinations thereof. The chemical-sensitive selective coating 34 may be applied, for example, with techniques such as dipping, spraying, or dispensing the coating on at least a portion of one or more of the cantilevered probes 30. The chemical-sensitive coating material may be applied onto a portion of one or more of the cantilevered probes 30 with the use of stencil masks or photomasks and photolithographic patterning techniques. The chemical-sensitive selective coating 34 may be applied in conjunction with photolithographic patterning, for example, using standard sputtering and other deposition techniques known in the art.

Multiple masking sequences can be used to apply multiple coating materials. Alternatively, multiple-component chemical-sensitive selective coatings 34 may be used. The multiple-component chemical-sensitive selective coatings 34 can comprise, for example, non-homogeneous coating materials, which can be applied in such a way that variations in coating thickness and/or composition occur when the materials are deposited.

When exposed to the explosive material 16 or to the target chemical species 12, one or more of the cantilevered probes 30 in the cantilevered probe array 20 may undergo an electrical or a mechanical shift, such as a shifted resonant frequency, a shifted Q factor, a shifted impedance, a shifted phase, or a shifted deflection amplitude. The cantilevered probe array 20 may include one or more reference cantilevered probes 30 r to provide a reference cantilevered probe response when the cantilevered probe array 20 is exposed to the explosive material 16 or to the target chemical species 12. The reference cantilevered probes 30 r may be uncoated, coated with an inert material, or otherwise protected from exposure to the explosive material 16 and the target chemical species 12.

FIG. 3 is a plan view of a pair of cantilevered probes 30 with probe heaters 36 and piezoelectric elements 32 for detecting an explosive material, in accordance with some embodiments of the present invention. As shown, one or more probe heaters 36 are disposed on or formed in the cantilevered probes 30. The probe heater 36 heats the cantilevered probes 30, for example, to initialize the cantilevered probes 30 prior to exposing the cantilevered probe array 20 to the explosive material. The probe heaters 36 also can be used to burn off, deflagrate, or otherwise react an explosive material that is adsorbed onto a surface of the cantilevered probe 30.

The piezoelectric elements 32, which may also serve as piezoelectric drive mechanisms for the cantilevered probes 30, can be configured to detect an explosive material adsorbed onto the cantilevered probe 30 when the cantilevered probe 30 is heated by the probe heater 36 to cause, for example, an exothermic reaction. In one example, the piezoelectric element 32 generates a piezoelectric element output signal when the probe heater 36 heats the adsorbed explosive material and an exothermic reaction or a phase change occurs. In another example, the piezoelectric element 32 detects an increase in temperature of the cantilevered probe 30 when an exothermic reaction occurs. In another example, the piezoelectric element 32 detects a shift in bending of the cantilevered probe 30 when the explosive material is adsorbed onto the cantilevered probe 30, is desorbed from the cantilevered probe 30, or reacts exothermically on the cantilevered probe 30. In another example, the piezoelectric element 32 detects a shift in a resonant frequency of the cantilevered probe 30 when the explosive material is adsorbed, is desorbed or exothermically reacts. In another example, the piezoelectric element 32 detects an impedance shift when the explosive or non-explosive material is adsorbed, is desorbed or exothermically reacts.

As show in FIG. 3, the cantilevered probe 30 can include a base end 26 and a tip 28. The cantilevered probe array 20 may be attached to a common base such as a substrate 22. The cantilevered probe 30 may have a rectangular shape, although other shapes may be suitably used, such as pointed shapes, V-shapes, triangular-shapes, or dual-arm shapes. A treated portion, such as the selective coating 34 disposed on at least a portion of the cantilevered probe 30, may aid in discriminating between various explosive materials and other target chemical species 12. In some embodiments, the cantilevered probe 30 is attached at each end, with the center of the cantilevered probe 30 free to vibrate. In another embodiment, the cantilevered probe 30 is attached on all sides in a diaphragm or membrane configuration.

A drive mechanism, such as the piezoelectric element 32 serving as a piezoelectric drive mechanism or a separate piezoelectric drive element, can be coupled to the cantilevered probe 30. The piezoelectric element 32 and/or the drive mechanism may comprise, for example, a patterned thin film of zinc oxide, PZT or aluminum nitride on a surface of the cantilevered probe 30. A sense mechanism may also be coupled to the cantilevered probe 30. The sense mechanism may comprise, for example, a piezoresistor attached to or formed in the cantilevered probe 30.

The probe heater 36 can be coupled to the cantilevered probe 30. The probe heater 36 can comprise, for example, a probe heater formed in or on the cantilevered probe 30. In addition to initiating an exothermic reaction or a phase change, the probe heater 36 may be used to heat the cantilevered probe 30 to an elevated temperature that initializes or re-initializes the treated portion or the selective coating 34. Alternatively, an external probe heater such as a heat lamp or a hot gas system may be used to heat and re-initialize the cantilevered probe 30. Chemical re-initialization may be accomplished, for example, by using cleaning processes or by reversing any chemical reactions that occurred on the treated portion.

Multiple cantilevered probes 30 may be arranged in a cantilevered probe array 20, the cantilevers being all identical, all different, or some combination thereof. The cantilevered probes 30 of a cantilevered probe array 20 may be driven and sensed, for example, with a piezoelectric drive element coupled to each cantilevered probe 30. In one embodiment, the piezoelectric elements in the array are connected in series. The series-connected piezoelectric elements in the array may be driven with as few as two electrical connections to the piezoelectric element array. Scanning the drive voltage through a range of frequencies can excite and sense one cantilevered probe 30 at a time, allowing interrogation of any cantilevered probe 30 in the array while minimizing the number of electrical connections required. In another configuration, the piezoelectric elements in the array are connected in parallel, such that as few as two electrical connections may be used to drive and sense cantilevered probes 30. In this configuration, failure of one cantilevered probe 30 does not prevent others from operating. In another configuration, the array of piezoelectric elements is connected in a series-parallel arrangement.

FIG. 4 is a schematic diagram of a system for detecting an explosive material, in accordance with some embodiments of the present invention. As shown, the chemical detection system 10 includes one or more cantilevered probes 30, one or more probe heaters 36 thermally coupled to the cantilevered probes 30, and one or more piezoelectric elements 32 disposed on the cantilevered probes 30. A controller 50 and an interface circuit 40 may be connected to one or more of the self-sensed cantilevered probes 30 configured in the self-sensed cantilevered probe array 20. The controller 50, which can be connected to the interface circuit 40, can be configured to drive and sense a plurality of self-sensed cantilevered probes 30 in the cantilevered probe array 20. It should be observed that, in some embodiments, the cantilevered probe array 20 may be electrically connected to the interface circuit 40 with as few as two cantilevered probe array drive pads 24. At least one cantilevered probe 30 in the cantilevered probe array 20 may exhibit a shifted cantilevered probe response when the cantilevered probe array 20 is exposed to an explosive material 16 or a target chemical species 12 and the cantilevered probe array 20 is actuated by the interface circuit 40. The piezoelectric element 32 can generate a piezoelectric element output signal that may be analyzed by the controller 50.

The interface circuit 40 can be configured to actuate the cantilevered probe array 20 with an excitation voltage applied to a piezoelectric material such as piezoelectric element 32 disposed on each cantilevered probe 30 in the cantilevered probe array 20. In one example, the interface circuit 40 includes an adjustable frequency generator that is scanned through a predetermined frequency range. In another example, the interface circuit 40 includes an impedance analyzer that is scanned through a resonant frequency of one or more cantilevered probes 30 in the cantilevered probe array 20. In another example, the interface circuit 40 includes an oscillator circuit operating at a resonant frequency of at least one cantilevered probe 30 in the cantilevered probe array 20. In another example, the interface circuit 40 includes an oscillator circuit operating at a predetermined frequency that is set to be off-resonance with respect to at least one cantilevered probe 30 in the cantilevered probe array 20. In another example, the interface circuit 40 includes control circuitry to monitor the amplitude of bending and vibration as the cantilevered probe 30 strikes against a fixed or adjustable mechanical stop. In another example, the interface circuit 40 comprises an impulse circuit for applying an electrical impulse to all of the cantilevered probes 30 in the cantilevered probe array 20. In another example, the interface circuit 40 or the controller 50 includes a fast Fourier transform (FFT) generator to perform a fast Fourier transform on the shifted cantilevered probe response. The interface circuit 40 can be configured to detect a shifted cantilevered probe response from one or more actuated cantilevered probes 30, such as a shifted resonant frequency, a shifted Q factor, a shifted impedance, a shifted phase, or a shifted deflection amplitude.

The controller 50 may receive a shifted cantilevered probe response from a set of one or more cantilevered probes 30 in the cantilevered probe array 20. The explosive material 16 or other target chemical species 12 may be determined, for example, based on the shifted cantilevered probe response. For example, the explosive material 16 may be determined based on a comparison between the shifted cantilevered probe response and a reference set of cantilevered probe responses. The reference set of cantilevered probe responses can comprise, for example, a learned set obtained during the calibration of the chemical-sensing system or from a statistical database of cantilevered probe responses.

To cancel out common mode effects such as temperature, one cantilevered probe 30 in the cantilevered probe array 20 may be a reference cantilevered probe 30 r, wherein the reference cantilevered probe 30 r provides a reference cantilevered probe response when the cantilevered probe array 20 is exposed to the explosive material 16 or the target chemical species 12.

In some embodiments, the explosive material 16 and/or other target chemical species 12 are adsorbed onto the cantilevered probe 30 by exposing the cantilevered probe 30 to an environment containing the explosive material 16 and/or the other target chemical species 12. To increase the rate of adsorption, transport mechanisms and concentrators may be added to the chemical detection system 10.

Using heat generated by the onboard probe heater 36 or an external probe heater thermally coupled to the cantilevered probe 30, the adsorbed explosive material 16 may ignite, deflagrate or otherwise burn. The chemical detection system 10 may include a thermally conductive mesh 58 substantially surrounding the cantilevered probes 30 to limit the egression of thermal energy from an exothermic reaction. A piezoelectric element 32 disposed on the cantilevered probe 30 can generate, for example, a piezoelectric element output signal when an exothermic reaction occurs. Alternatively or in addition, the piezoelectric element 32 may also serve as a piezoelectric drive mechanism and a piezoelectric sense mechanism that senses the explosive material 16 by detecting bending or shifts in a resonant frequency of the cantilevered probe 30.

Detection of a non-explosive material or other target chemical species 12 adsorbed onto a surface of the cantilevered probe 30 may be accomplished, for example, using characteristic bending shifts, frequency shifts, exothermic or non-exothermic reaction indicators, phase change indicators, impedance shifts, or a combination thereof. Specificity and delineation of the explosive material 16 and other target chemical species 12 may be increased with selective coatings applied to one or more of the cantilevered probes 30 in the cantilevered probe array 20.

FIG. 5 a, FIG. 5 b and FIG. 5 c show characteristic resonant frequencies of a cantilevered probe prior to exposure to an explosive material, after exposure to the explosive material, and after deflagration, ignition, or evaporation of the explosive material, respectively, in accordance with some embodiments of the present invention. An exemplary response of a cantilevered probe with resonant frequency 90 a is seen in FIG. 5 a. As the explosive material is adsorbed onto the cantilevered probe, the resonant frequency decreases with mass loading indicated by shifted resonant frequency 90 b, as seen in FIG. 5 b. After the explosive material deflagrates, ignites, or otherwise desorbs from the cantilevered probe, the response curve with resonant frequency 90 c returns towards the resonant frequency 90 a, as seen in FIG. 5 c. Time dependencies of the frequency shifts prior to, during, or after cantilevered probe heating may provide characteristics associated with various absorbed and desorbed explosive and non-explosive materials. Frequency shifts with the application of predetermined cantilever heating profiles may also provide characteristic signatures for the adsorbed materials.

FIG. 6 a, FIG. 6 b and FIG. 6 c show characteristic bending of a cantilevered probe prior to exposure to an explosive material, after exposure to the explosive material, and after deflagration or ignition of the explosive material, respectively, in accordance with some embodiments of the present invention. In some embodiments, the cantilevered probe 30 initially has a tip 28 that is essentially straight, as seen in FIG. 6 a. With exposure to and adsorption of the explosive material 16 or other target chemical species 12 onto a surface of the cantilevered probe 30, the probe may remain neutral, bend upwards, or bend downwards depending on the stress state of the cantilevered probe 30, with the tip 28 deflecting an amount equal to a displacement 92 b, as seen in FIG. 6 b. When the cantilevered probe 30 is heated with an onboard or external probe heater, the explosive material 16 may deflagrate, ignite, or otherwise desorb from the surface of the cantilevered probe 30, allowing the cantilevered probe 30 to return towards the initial, undeflected state with the tip 28 back in a neutral position, as seen in FIG. 6 c. It should be noted that localized heating of the cantilevered probe 30 may contribute to beam bending, as thermal gradients across the cantilever produce moments that can cause bending.

FIG. 7 a, FIG. 7 b and FIG. 7 c illustrate simultaneous bending and vibration of a cantilevered probe prior to exposure to an explosive material, after exposure to the explosive material, and after deflagration or ignition of the explosive material, respectively, in accordance with some embodiments of the present invention. The cantilevered probe 30 vibrates at resonant frequency 90 a about a neutral position prior to mass loading, as seen in FIG. 7 a. With the addition of explosive material on the cantilevered probe 30, the tip may deflect an average amount equal to a displacement 92 b while vibrating at a shifted or unshifted resonant frequency 90 b, as seen in FIG. 7 b. After deflagration, ignition, or desorption of the explosive material from the cantilevered probe 30, the tip may return towards the initial, undeflected state while vibrating at a resonant frequency 90 c, as seen in FIG. 7 c.

FIG. 8 a, FIG. 8 b and FIG. 8 c illustrate periodic heating of a cantilevered probe prior to exposure to an explosive material, during exposure to the explosive material, and after deflagration or ignition of the explosive material, respectively, along with a generated piezoelectric element output signal, in accordance with some embodiments of the present invention. A piezoelectric element may generate a relatively small peak during each periodic heating cycle 94 a of the cantilevered probe and return to a low level as the cantilevered probe cools, as indicated by the piezoelectric element output signal 96 a in FIG. 8 a. As the explosive material deposits and is adsorbed onto the cantilevered probe, the piezoelectric element output signal 96 a replicates relatively small peaks during each periodic heating cycle 94 b, until sufficient explosive material is adsorbed so that the explosive material deflagrates and ignites or otherwise combusts, thereby generating a high-level piezoelectric element output signal 96 b corresponding to the energy released by the exothermic reaction, as seen in FIG. 8 b. Other mechanisms such as melting or evaporation may provide piezoelectric element output signals 96 b with higher, lower, or time-dependent characteristics different from that shown. As the cantilevered probe cools down from the energy release, the piezoelectric element output signal 96 c generally decreases towards a baseline with relatively small peaks coinciding with periodic heating cycles 94 c applied to the cantilevered probe, as indicated in FIG. 8 c. Heat pulses of the periodic heating cycles may be tailored, for example, to allow the cantilevered probe to reach characteristic melting, evaporation, and deflagration temperatures associated with a given explosive material.

FIG. 9 illustrates an array of cantilevered probes with an explosive material concentrator surrounding the cantilevered probe array, in accordance with some embodiments of the present invention. One or more selective coatings 34 are optionally applied to the cantilevered probe array 20 having a plurality of self-sensed cantilevered probes 30. In the example shown, the cantilevered probes 30 a, 30 b, . . . 30 n are selectively coated with selective coatings 34 a, 34 b, . . . 34 n, respectively. The reference cantilevered probe 30 r is shown with no coating.

In this example, the cantilevered probes 30 a, 30 b, . . . 30 n are nominally the same size and thickness. Frequency differentiation for this set of cantilevered probes can be achieved by varying the area of the cantilevered probes that is covered by the coating. Different amounts of selective coating material can be disposed on each cantilevered probe, varying the effective mass of each cantilevered probe and changing the resonant frequencies accordingly. Piezoelectric elements 32 a, 32 b, . . . 32 n and probe heaters 36 a, 36 b . . . 36 n on the cantilevered probes 30 a, 30 b, . . . 30 n and 30 r, respectively, may be coated, partially coated, or uncoated with the selective coatings 34.

An explosive material concentrator 68 can be coupled to one or more cantilevered probes 30. The concentrator 68, such as a condenser and heater system, may be included to concentrate the explosive material and/or other target chemical species 12 proximal to the cantilevered probe array 20 for detection. In some embodiments, the concentrator 68 with one or more heaters 68 a, 68 b, 68 c and 68 d surrounding the cantilevered probe array 20 is heated after the explosive material is adsorbed thereon, increasing the concentration of the explosive material in the vicinity of the cantilevered probes 30 and allowing a higher adsorption rate of the explosive material onto one or more of the cantilevered probes 30. The heaters 68 a, 68 b, 68 c and 68 d that surround the cantilevered probe array 20 can comprise, for example, discrete heaters, integrated resistive heaters, or integrated circuitry.

FIG. 10 illustrates a handheld system for detecting an explosive material, in accordance with some embodiments of the present invention. As shown, the handheld system 70 includes an enclosure 60, one or more cantilevered probes 30 within the enclosure 60, probe heaters 36 thermally coupled to the cantilevered probes 30, and piezoelectric elements 32 disposed on the cantilevered probes 30. One or more of the cantilevered probes 30 in the cantilevered probe array 20 have probe heaters 36 to locally heat selected cantilevered probes 30. Piezoelectric elements 32 can be configured to detect an explosive material 16 adsorbed onto the cantilevered probes 30 when the probe heaters 36 heat the cantilevered probes 30.

In some embodiments, the piezoelectric element 32 also serves as a piezoelectric drive and as a piezoelectric sense mechanism. The piezoelectric element 32 can detect the explosive material 16 adsorbed onto one or more of the cantilevered probes 30. One or more selective coatings 34 may be applied to one or more of the cantilevered probes 30 in the cantilevered probe array 20. An interface circuit 40 may be coupled to the cantilevered probe array 20. The enclosure 60 can have an inlet port 62 to allow ingression of the explosive material 16 into the enclosure 60 and an outlet port 64 to allow egression of the explosive material 16 or a byproduct thereof from the enclosure 60. When the cantilevered probe array 20 is exposed to the explosive material 16 and the interface circuit 40 actuates the cantilevered probe array 20 during or after heating, one or more of the cantilevered probes 30 in the cantilevered probe array 20 may exhibit a response such as a resonant frequency shift, a shift in bending, a thermal signature, a recoil response such as an impulse followed by ring down, a pyroelectric charge generation, an impedance shift, a temperature shift, or a combination thereof.

The cantilevered probe array 20 may include a plurality of cantilevered probes 30 that are frequency-differentiated. The plurality of cantilevered probes 30 in the cantilevered probe array 20 may be electrically connected to a single pair of cantilevered probe array drive pads, and one or more groups of cantilevered probes 30 may be included within the enclosure 60.

The handheld system 70 may include a controller 50 in communication with the interface circuit 40. The controller 50 can be configured to receive a shifted cantilevered probe response and piezoelectric element output signals from a set of cantilevered probes 30 in the cantilevered probe array 20. The shifted cantilevered probe responses and the piezoelectric element output signals can be analyzed and used to determine the constituency and concentration of the explosive material 16.

The cantilevered probe array 20 may include a reference cantilevered probe 30 r. The reference cantilevered probe 30 r may provide a reference cantilevered probe response when the cantilevered probe array 20 is exposed to the explosive material 16 and the target chemical species 12.

The handheld system 70 may include a thermally conductive mesh 58 such as a copper or metal screen substantially surrounding cantilevered probes 30, such as to limit the egression of thermal energy from an exothermic reaction when the probe heater heats the cantilevered probe. The handheld system 70 also may include a transport mechanism 66 such as a pump, fan or blower and ductwork or piping for transporting the explosive material 16 and/or the target chemical species 12 to the cantilevered probe array 20. The handheld system 70 may include a concentrator 68 such as a compressor or a condenser to concentrate the explosive material 16 proximal to one or more of the cantilevered probes 30 in the cantilevered probe array 20. In some embodiments, one or more heaters of the concentrator 68 are located near the cantilevered probe array 20 so that the explosive material 16 is concentrated on one or more of the cantilevered probes 30 when the concentrator 68 is locally heated to desorb the explosive material collected by the concentrator 68.

Command and data entry input devices such as buttons, keypads, or softkeys, can be incorporated to allow the selection of functions and operation of the handheld system 70. Results of measurements can be displayed on an output device, such as an LCD, or communicated to another analysis system through a wired communication port such as a universal serial bus (USB) port or through a wireless communication protocol.

FIG. 11 is a flow chart of a method for detecting an explosive material, in accordance with some embodiments of the present invention. The chemical detection method can include various steps to detect and identify one or more explosive materials and/or target chemical species, such as with a self-sensed cantilevered probe array that includes a piezoelectric element disposed on one or more cantilevered probes in the cantilevered probe array.

The cantilevered probes in the cantilevered probe array may be frequency-differentiated, separated in the frequency domain such that any one of the cantilevered probes can be measured independently of the others using, for example, a frequency generator, a frequency synthesizer, a controlled oscillator, or an impedance analyzer when the cantilevered probes are configured in series or in parallel with other cantilevered probes. The cantilevered probe array includes, for example, at least two-series connected cantilevered probes electrically connected to a pair of cantilevered probe array drive pads. Alternatively, the cantilevered probe array may include at least two parallel-connected cantilevered probes electrically connected to a pair of cantilevered probe array drive pads. Alternatively, the cantilevered probe array may include a network of series-connected and parallel-connected cantilevered probes electrically connected to a pair of cantilevered probe array drive pads. One or more groups of cantilevered probes may be connected to the same set of cantilevered probe array drive pads or to a different set of cantilevered probe array drive pads on the same substrate for external connection to an interface circuit.

The cantilevered probe array may include one or more selective coatings applied to one or more cantilevered probes in the cantilevered probe array. Exemplary chemical-sensitive coating materials include an epoxy resin, a fluoropolymer, a gold layer, a palladium layer, an alcohol-absorbent polymer, a water-absorbent material, a chemical-sensitive polymer, a chemical-sensitive layer, a biosensitive material, a thiol, and derivatives and combinations thereof. The selective coating can be applied, for example, by standard deposition techniques such as sputter depositions, electron beam depositions, or plasma-enhanced chemical vapor depositions, or by dipping, spraying or dispensing the coating material onto at least a portion of one or more cantilevered probes. In another example, a chemical-sensitive selective coating is applied to one or more cantilevered probes with a stencil mask and the selective masking of one or more cantilevered probes. A single material may be applied through the mask.

A plurality of chemical-sensitive coating materials may be applied to a set of cantilevered probes in the cantilevered probe array. For example, multiple masks may be used for multiple coatings with different coating materials on selected portions of one or more cantilevered probes. Alternatively, coating with multiple materials through a single mask may be accomplished by spraying a non-homogenous coating material onto a set of cantilevered probes in the cantilevered probe array such that cantilevered probes in the array are coated with differences in coating constituency, thickness, or fraction of coverage.

A probe heater on or near the cantilevered probe can be thermally coupled to at least one cantilevered probe, which may be heated to initialize the cantilevered probe prior to exposing it to the explosive material or to initiate an exothermal reaction. For example, the probe heater can be used to locally heat the cantilevered probe to an elevated temperature to evaporate, burn off, or otherwise remove material from the surfaces of the cantilevered probe.

The cantilevered probe array may be initialized, as seen at block 100. Initialization of the array can be accomplished, for example, by running a scan through the resonant frequencies of the cantilevered probes in the cantilevered probe array to establish a baseline or to ensure that all the cantilevered probes and the interface electronics are functioning properly.

Explosive material can be exposed to and adsorbed onto one or more cantilevered probes, as seen at block 102. For example, the self-sensed cantilevered probe array can be exposed to an explosive material. A valve and associated piping may be used to expose the cantilevered probe array to the explosive material and a carrier. The explosive material may be transported to the cantilevered probe array using, for example, fans, blowers, or pumps to force flow of the explosive material and a carrier gas or liquid onto the cantilevered probe array. Convective processes or normal diffusive processes due to concentration gradients may be used, for example, to transport the explosive material to the cantilevered probe array for detection.

An explosive material, such as 2,4,6-trinitrotoluene (TNT), 2,4,6, n-tetranitro-n-methylaniline (Tetryl), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), pentaerythritol tetranitrate (PETN), glycerol trinitrate (nitroglycerin), ethylene glycol dinitrate (EGDN) or derivatives or combination thereof, can be adsorbed onto one or more cantilevered probes in the cantilevered probe array.

The explosive material may be concentrated near or on the cantilevered probe array. Concentration of the explosive material may be accomplished, for example, with a compressor and a valve system to increase the pressure in the vicinity of the cantilevered probe array. A condenser and a heater may be used, for example, to collect samples of the explosive material and then release the explosive material in proximity to the cantilevered probe array. In some embodiments, a concentrator with one or more heating elements surrounding a cantilevered probe array is heated locally after an explosive material is adsorbed thereon, increasing the concentration of explosive material in the vicinity of the cantilevered probes and allowing a higher adsorption rate of explosive material onto one or more of the cantilevered probes.

The cantilevered probe can be heated to cause, for example, an exothermic reaction or a phase change with the adsorbed explosive material, as seen at block 104. The probe heaters coupled to one or more cantilevered probes may be heated to react the explosive material. The piezoelectric element may generate a piezoelectric element output signal when the explosive material is reacted. Alternatively, the reaction of the explosive material can result in the volatile material being desorbed from a cantilevered probe, which causes a shift in the resonant frequency of the cantilevered probe due to its decreased mass. Alternatively, the reaction of the explosive material may result in a formation of a material on the surface of the cantilevered probe that increases the vibrational stiffness of the cantilevered probe and produces a frequency shift. Alternatively, reaction of the explosive material may result in a stressed film on the surface of the cantilevered probe that causes a static deflection of the cantilevered probe. The static deflection can be measured, for example, with a tapping mode where the cantilevered probe is tapped against a reference surface at a fixed distance away from the cantilevered probe, or with a tapping mode where the cantilevered probe is tapped against an adjustable mechanical stop that is adjusted so the cantilevered probe has a consistent amount of contact with the mechanical stop. Alternatively, recoil of the cantilevered probe when the adsorbed explosive material is ignited or deflagrated may produce an impulse response with a ring-down characteristic to identify the event.

A piezoelectric element output signal can be detected, as seen at block 106. The piezoelectric element output signal generated by the piezoelectric element can be detected, for example, with an analog-to-digital converter or a threshold detector. To validate the measurement, additional cantilevered probe responses may be detected. A cantilevered probe response may be detected, for example, from at least one self-sensed cantilevered probe in the cantilevered probe array by actuating one or more cantilevered probes.

In some embodiments, an exposed cantilevered probe array is actuated by applying an excitation voltage to a piezoelectric material disposed on each cantilevered probe in the cantilevered probe array. The exposed cantilevered probe array can be actuated with a signal generator or a frequency generator by scanning the cantilevered probes through a predetermined frequency range, allowing the resonant frequencies of one or more cantilevered probes to be determined. In another example, the exposed cantilevered probe array is actuated by driving the exposed array at a resonant frequency of one cantilevered probe in the cantilevered probe array, then switching as desired to a resonant frequency of another cantilevered probe for additional measurements. In another example, the exposed cantilevered probe array is actuated by driving the exposed array at a predetermined frequency, wherein the predetermined frequency is off-resonance with respect to at least one cantilevered probe in the cantilevered probe array. In another example, the amplitude of vibration is controlled as the cantilevered probe strikes against a fixed or adjustable mechanical stop. In another example, the exposed cantilevered array is actuated with an electrical impulse applied to the cantilevered probe array.

The piezoelectric element output signal can be analyzed, as seen at block 108. Analyzing the piezoelectric element output signals and the cantilevered probe response from one or more actuated cantilevered probes comprises, for example, measuring a shifted resonant frequency, a shifted Q factor, a shifted impedance, a shifted phase, a shifted deflection amplitude, or a combination thereof and comparing the responses to known or calibrated responses. A fast Fourier transform (FFT) may be performed on the cantilevered probe responses from one or more actuated cantilevered probes. The entire array of cantilevered probes, a subset thereof, or an individual cantilevered probe may be addressed by selective actuation and detection. With the availability of a reference cantilevered probe, a reference cantilevered probe response may be detected from one or more reference cantilevered probes in the cantilevered probe array. The explosive material may be determined based on comparing a measured shift from one or more actuated cantilevered probes to a reference set of cantilevered probe responses, and determining the explosive material based on the reference set of cantilevered probe responses.

The explosive material can be determined, for example, based on the piezoelectric element output signal from a piezoelectric element disposed on the cantilevered probe, as seen at block 110. Alternatively or in addition to, a non-explosive material may be adsorbed onto the surface of the cantilevered probe and determined. Determining the explosive material, non-explosive material or other target chemical species may include, for example, analyzing the piezoelectric element output signal and other cantilevered probe responses such as a resonant frequency shift of the cantilevered probe, a shift in bending of the cantilevered probe, a thermal signature, a recoil response, a pyroelectric charge generation, an impedance shift, a temperature shift, or a combination thereof.

To determine the explosive material or other target chemical species, the self-sensed cantilevered probe array may be scanned through a predetermined frequency range. When activated, for example, with an interface circuit that scans through the resonant frequencies of one or more cantilevered probes, each cantilevered probe, in turn, may be excited and oscillated by the interface circuit as the frequency of the oscillator or frequency generator is scanned through each resonant frequency. Depending on the type and amount of a explosive material and the coating on the cantilevered probe, the cantilevered probes in the array may exhibit shifted cantilevered probe responses such as a shifted resonant frequency, a shifted Q factor, a shifted impedance, a shifted phase, a shifted deflection amplitude, or a combination thereof.

Temperature measurements from one or more probe heaters serving as a temperature sensor or other on-board temperature sensors may be used to indicate the temperature of the heated cantilevered probe, from which the ignition temperature of the explosive material can be determined. Characteristic properties such as the heat of vaporization, melting temperature, phase change, chemical reactions, exothermic reactions, or endothermic reactions associated with adsorbed explosive material and other target chemical species may be interpreted to aid in the determination of the explosive material or target chemical species.

A controller or a software application running on a computer or digital device may be used to analyze the cantilevered probe responses and determine one or more components and their concentration in the sample. The explosive material may be determined in part based on the detected reference cantilevered probe response, for example, by a common mode correcting for effects such as temperature, pressure and viscosity of the sampled medium. The detected explosive material or target chemical species may include, for example, mercury, hydrogen, an alcohol, water vapor, a chemical element, a chemical compound, an organic material, an inorganic material, a gaseous substance, a liquid, a biological material, a DNA strand, a bioactive agent, a toxin, and derivatives and combinations thereof.

Using pattern recognition, modeling functions or signal processing techniques such as fuzzy logic, the explosive material may be determined based on comparing a measured shift from one or more actuated cantilevered probes to a reference set of cantilevered probe responses, and determining the explosive material based on the reference set of cantilevered probe responses. The reference set of cantilevered probe responses may comprise, for example, a learned set from calibration runs or from a statistical database with expectation values for various explosive materials and target chemical species.

Having illustrated and described the principles of the invention in exemplary embodiments, it should be apparent to those skilled in the art that the illustrative embodiments can be modified in arrangement and detail without departing from such principles. In view of the many possible embodiments to which the principles of the invention can be applied, it should be understood that the illustrative embodiments are intended to teach these principles and are not intended to be a limitation on the scope of the invention. We therefore claim as our invention all that comes within the scope and spirit of the following claims and their equivalents. 

1. A chemical detection system for detecting a target material, comprising: a cantilevered probe; a probe heater thermally coupled to the cantilevered probe; and a piezoelectric element disposed on the cantilevered probe, further comprising a detection circuit coupled to the piezoelectric element of the cantilevered probe, wherein the piezoelectric element and detection circuit are configured to detect a movement of the cantilevered probe based on signals generated by the piezoelectric element as it moves.
 2. The system of claim 1, wherein the piezoelectric element is configured to detect a temperature change of the cantilevered probe.
 3. The system of claim 1, wherein the piezoelectric element is configured to detect an impedance shift of the cantilevered probe.
 4. The system of claim 1, wherein the piezoelectric element is configured to detect a resonant frequency shift of the cantilevered probe.
 5. The system of claim 1, wherein the piezoelectric element comprises a piezoelectric film disposed on a surface of the cantilevered probe.
 6. The system of claim 1, wherein the cantilevered probe is part of a cantilevered probe array.
 7. The system of claim 6, wherein a selective coating is disposed on one or more cantilevered probes in the cantilevered probe array.
 8. The system of claim 1, wherein the probe heater comprises a piezoresistive element formed in the cantilevered probe, a heater element disposed on the cantilevered probe, or both.
 9. The system of claim 1, wherein the piezoelectric element comprises zinc oxide, lead zircanate titanate, aluminum nitride, a piezoelectric material, or a derivative or combination thereof.
 10. The system of claim 1, wherein the piezoelectric element comprises a pyroelectric material.
 11. The system of claim 1 further comprising: an interface circuit electrically coupled to the piezoelectric element.
 12. The system of claim 1 further comprising: an interface circuit comprising the piezoelectric element as a bridge element in an AC bridge circuit.
 13. The system of claim 12, wherein the AC bridge circuit is tuned to one of an on-resonance condition or an off-resonance condition to detect the target material.
 14. The system of claim 6, wherein at least one cantilevered probe in the cantilevered probe away is tuned to an on-resonance condition and at least one other cantilevered probe in the cantilevered probe away is tuned to an off-resonance condition.
 15. The system of claim 6, wherein at least two cantilevered probes in the cantilevered probe away are frequency-differentiated.
 16. The system of claim 15, wherein piezoelectric elements disposed on the at least two cantilevered probes in the cantilevered probe away are connected in series. 